This invention relates to dowel and tying bars, and to construction joints for transferring stresses across a joint between concrete constructions.
Concrete responds to changes in temperature and moisture when movement associated with these changes (or for other reasons such as internal chemical reaction) is restrained, stresses develop that can lead to cracking. To control cracking, joints are built at intervals short enough to hold stresses below critical values. Transverse joints are saw cut, placed through induced cracking, or formed at spacings from 12 to 40 feet for a typical concrete highway pavement.
Concrete pavements for highways, airport runways and the like are generally placed in strips or lanes with a longitudinal joint formed between adjacent strips or lanes. Concrete is poured in the first strip and allowed to cure. Subsequently, concrete is poured and cured in the adjacent strip and so on until the concrete pavement is completed. A longitudinal joint is formed between adjacent strips to facilitate construction and to reduce stresses and control cracking caused by contraction or expansion of the concrete.
Similarly, joints are formed in concrete structural slabs, walls, footings and the like to minimize stresses and/or simplify construction methods. Of these joints, there are several types. The contraction joint minimizes uncontrolled cracking due to changes in temperature and moisture. The expansion joint provides a space between slabs to allow for expansion or swelling of the slab as temperature and moisture increase or growth due to any cause occurs. A construction joint provides a finished edge or end so that construction operations interrupted for some length of time may be continued or resumed without serious structural penalty.
Load is transferred across a joint principally by shear. Some bending moment may be transferred across the joint through tied joints that remain closed. Good load transfer capability must be built into the joint, or the joint will wear and the load carrying ability of the concrete slab or structure will be reduced. The alternative is to strengthen the concrete by improving support or increasing depth to minimize the joint load transfer weakness.
Tie bars and dowels are often used in concrete design to improve load transfer at the joint between concrete slabs or structures. Such tie bars and dowels are embedded in the concrete and arranged across the joint in a direction substantially perpendicular to the axis defined by the joint. Various approaches, depending on the type of tie bar or dowel, have been suggested with respect to concrete construction joints such as set forth in the disclosure of the following patents:
______________________________________ Reg. No. Inventor Title ______________________________________ 2,106,095 Heltzel Expansion Joint 2,299,670 Wescott Dowel Bar Structure 2,358,328 Heltzel Joint 2,494,869 Godwin Dowel Assembly for Concrete Road Joints 2,575,247 Carter Sealed Joint for Concrete Slab Road Pavement 3,397,626 Kornick et al. Plastic Coated Dowel Bar for Concrete 3,559,541 Watstein Concrete Joint Load Transfer Device 3,698,292 Koester Expansion Gap Sealing Device 4,449,844 Larsen Dowel for Pavement Joints ______________________________________
The functions of the tie bars and dowels are to keep contiguous sections of concrete in alignment during contraction and re-expansion, and to transfer shear stresses and bending moments across the joint between the two slabs. The prior art dowels are often made smooth, lubricated, or coated entirely with plastic as disclosed in Kornick et al., U.S. Pat. No. 3,397,626 to prevent the dowel from bonding to the concrete and allow the concrete slab or structure to slide relative to the dowel in a direction substantially perpendicular to the axis defined by the joint. Such movement of the slab relative to the dowel prevents build up of stress in the dowel that may result in cracking of the concrete.
In an alternative construction as disclosed in Larsen, U.S. Pat. No. 4,449,844, the dowel has its outer ends bonded to concrete and its central portion covered with plastic to prevent bonding to concrete. The dowel disclosed in Larsen performs a latent spring function to limit the movement of the concrete slab relative to the dowel when temperature changes cause the length of the slab section to vary with time.
A major disadvantage of the prior art dowels and tie bars is that the dowels and tie bars prevent movement of the concrete slab relative to an adjacent concrete slab in a direction substantially parallel to and aligned with the axis defined by the joint. In such situations, the dowels and tie bars provide enough restraint against movement and shrinkage so that the concrete slab or structure induces stresses along a line substantially defined by ends of the dowels or tie bars. This problem is most evident in the situation when adjacent concrete slabs or strips are placed and cured in repetitive order or when adjacent concrete slabs or structures are subjected to extreme temperature differences.
For example, it is well known that concrete typically shrinks after placement. If a second concrete paving slab is placed adjacent to a first concrete paving slab that has contracted from thermal and drying shrinkage, the second concrete paving slab will likewise attempt to shrink similar to the shrinkage of the first concrete paving slab. However, dowels and tie bars arranged across the joint between the first and second concrete paving slabs will restrain the second concrete paving slab from shrinking during curing. The developed internal stress in the second concrete paving slab can create an undesirable condition that may result in cracking. Even if cracks do not develop, the internal stresses are added to the stress from the normally applied design loads and could reduce the service life of the pavement. FIG. 1 illustrates cracks that could develop in such situations.
Many of the prior art joint constructions are preoccupied with resisting movement in two dimensions (dowel) or three dimensions (tie bar). The prior art dowels and tie bars do not restrain movement in one dimension and two dimensions, respectively, which would reduce the cracking problems associated with adjacent concretes moving relative to each other in a direction aligned with the joint axis and still maintain the outer surface of the contiguous sections of concrete in alignment during contraction and re-expansion. The present invention overcomes such prior art shortcomings.